If HI clouds exist in pressure equilibrium in an environment where gas pressure is a function of z-distance and if HI cloud density is a function of z-distance, it can be shown that a quantity called the Virial Measure is a function of z-distance. The Virial Measure is that distance at which a cloud would be in gravitational equilibrium if its internal kinetic temperature is indicated by profile linewidth. The Virial Measure is derived from observed cloud parameters and has been calibrated for clouds of known distance so that it can be used to determine the distance to other HI clouds. The magnitude of various terms in the virial equation can thus be derived for several hundred HI clouds. It is demonstrated that the strength of the interstellar magnetic field is a function of z-distance.